1. Technical Field
The present invention relates to thermal displacement correcting devices for correcting axial thermal displacement of a tool attached to the spindle of a machine tool.
2. Description of the Related Art
In machine-tool installations such as machining centers, the bearings that rotatably support the tool spindle run hot, such that the heat occurring due to the thermal emission is transferred to the spindle, and the tool chucked onto the spindle, and elongates the tool. This elongating of a tool degrades its machining accuracy.
Against this backdrop, to prevent such degradation to machining accuracy, correction methods—an example of which is disclosed in Japanese Unexamined Pat. App. Pub. No. 2004-148443—of correcting tool thermal displacement along the spindle axis orientation have been proposed to date.
The correcting method in JP 2004-148443 is performed by: measuring a first machine-tool temperature near the spindle bearing, and a second machine-tool temperature of a region of the machine tool having high thermal stability, such as the bed; estimating thermal displacement (correction amount) of the tool axially from a predetermined formula for computation on the basis of the difference between the measured first machine-tool temperature and second machine-tool temperature; and correcting the position of the tool axially on the basis of the estimated thermal displacement (correction amount). Performing correction in this ways allows canceling out of the tool thermal displacement to enable machining of a workpiece while preventing degradation in the machining accuracy.
Nevertheless, machine tools have tool magazines in which tools are stowed, and are also provided with a tool changer for exchanging a tool attached to the spindle with a tool stowed in the tool magazine. The machining of workpieces is performed using various tools, with the tool changer exchanging the tool attached to the spindle with a tool in the tool magazine.
Furthermore, a tool removed from the spindle and stowed in the tool magazine gradually cools, eventually arriving at a temperature at the same level as room temperature; but cooling to the room-temperature level takes a given amount of time. Consequently, the tools stowed in the tool magazine each differ in temperature, depending on whether the tool, not having been chucked to the spindle, is not yet used, whether, since the tool was stowed in the tool magazine, the given amount of time has elapsed for the tool temperature to drop to the room-temperature level, or whether, since the tool was stowed in the tool magazine, the given amount of time has not elapsed and the tool temperature is in the midst of dropping.
Thus, the temperature at a start of spindle-chucking when the tool is chucked to the spindle by the tool changer differs according to the tool, and if the tool temperature at spindle-chucking start is different, the change in temperature (thermal displacement) of the tool thereafter will also vary.
Accordingly, to carry out a more rigorous thermal displacement correction, tool temperature at the start of chucking to the spindle must be taken into consideration in calculating thermal displacement of a tool. Nonetheless, in the method in which a first machine-tool temperature near the spindle bearing, and a second machine-tool temperature of a region of the machine tool having high thermal stability are measured to estimate the thermal displacement of the tool—i.e., the above-described conventional thermal displacement correction technique—this point is not taken into consideration, which is prohibitive of performing high-precision correction.
A further problem with this conventional solution is that sensors for measuring the first machine-tool temperature and the second machine-tool temperature are required, raising production costs and complicating the device configuration.